The present invention relates to a motor controller for spindle motors. In particular, the present invention relates to a digital motor controller that employs a frequency-locked loop to control the speed of a three-phase spindle motor for use in a hard disk drive.
The disks of a computer hard disk drive are typically rotated using three-phase brushless spindle motors. These motors generally have a stationary stator containing three phases connected in a “Y” configuration such that all phases share a common center tap. Opposite the center tap, each phase is connected to a terminal through which current is either supplied or sunk by drive circuitry connected thereto. These motors also have a rotatable rotor that contains a plurality of permanent magnet segments.
The disks are mechanically mounted to the rotor and are rotated by energizing selective phases of the stator to induce magnetic fields that interact with the permanent magnet segments in the rotor to cause the rotor and the disks to rotate in the desired direction at the desired operating speed. The selective energizing of phases in the stator in a predetermined sequence is known as commutation of the motor. In a classical six-state commutation mode, each commutation state is defined by one of the three phases being held at a high impedance while current is supplied to a second phase and sunk from a third phase.
The rotation of the motor induces a back electromotive force (“back EMF”) voltage at each of the three motor terminals. The back EMF voltage is a generally sinusoidal signal with a period proportional to the electrical rotation of the motor.
Commutation of the spindle motor is controlled by commutation circuitry. To maximize the spindle motor torque, the commutation circuitry attempts to drive the motor in time with the electrical rotation of the motor. This is typically accomplished through use of phase-locked loop circuitry based upon the induced back EMF voltage. The phase-locked loop circuitry compares this back EMF voltage at the unenergized terminal (i.e., the terminal held at a high impedance) to the voltage at the motor's center tap, which is representative of the average voltage of the three terminals. Next, a signal is generated indicative of the zero crossings of the back EMF voltage signal, that is, when the back EMF voltage changes polarity with respect to the voltage at the center tap. This zero crossings signal is then used by the commutation circuitry as a reference for commutation timing.
Pulse width modulation techniques are commonly used to drive, or energize, the hard disk drive spindle motor. A motor controller circuit varies the duty cycle of the driving waveforms to achieve a desired motor current waveform. To minimize unwanted acoustic noise, the motor controller circuit may drive a sinusoidal-shaped current through the motor. Unlike the classic six-state commutation mode in which one terminal is held at a high impedance, commutation using sinusoidal pulse width modulation generally requires that all three phases of the motor be driven simultaneously. Thus, with sinusoidal pulse with modulation, one terminal of the motor is driven to a high voltage while the other two terminals are modulated by being alternately driven to high and low voltages to shape the current through the motor.
It becomes more difficult to detect back EMF voltage when sinusoidal pulse width modulation is used to drive the spindle motor because all three terminals of the motor are simultaneously driven. Accordingly, it becomes necessary to predict an approximate location of the back EMF zero crossing on at least one of the three terminals, and stop driving, or float, the selected motor terminal long enough to detect the zero crossing. This act of floating a terminal is referred to as opening a window, and is generally performed by the motor controller circuitry.
Most phase-locked loops of conventional motor controllers are analog in nature. Such analog approaches, however, generally require expensive external components. Analog motor controllers also have a limited lock range, that is, a limited range of motor rotation rates to which they can synchronize the commutation circuitry. This may restrict the number of hard disk drive types that the motor controller can support.
Attempts to implement the motor controller with digital electronics have either required complex digital circuitry along with significant silicon die area or have sacrificed hard disk drive performance. One performance item that is often sacrificed is the resolution of the sinusoid used to sinusoidally pulse width modulate the motor, which may negatively affect the acoustic noise of the motor. Other performance items often sacrificed include the ability to avoid period jitter, which may negatively affect the read/write electronics, and the ability to reliably and efficiently lock to the motor.